Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/10—Concentrating spent liquor by evaporation
  • D21C11/106—Prevention of incrustations on heating surfaces during the concentration, e.g. by elimination of the scale-forming substances contained in the liquors
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D1/00—Evaporating
  • B01D1/26—Multiple-effect evaporating
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0035—Introduction of compounds, e.g. sodium sulfate, into the cycle in order to compensate for the losses of pulping agents

Definitions

• pulping processes chemical losses may be replaced by residual solutions from other processes either from the same factory or from other factories.
• Exemples of residual solutions which can be used to cover the chemical losses are splitting liquid from splitting of soap skimmings and residual acid from the production of chlorine dioxide.
• a splitting liquid which contains sodium sulphat and lignin.
• a residual acid solut is obtained which contains considerable quantitities of sodi sulphate and sulphuric acid.
• the residual acid solution can be used for splitting of the soap skimmings.
• the splitting liquid thus formed, and any excess of residual acid, can be used to cover the chemical losses in the sulphate process.
• both the splitting liquid and the residual acid solution hav very low dry matter content, usually less than 25%.
• the sodium sulphate content in the liquor will increase considerably during its passage through the evaporation plant.
• sodium sulphate has what is known as a reversed solubility, i.e. its solubility decreases with increasing temperature.
• splitting liquid and residual acid in this way as compared with the other known method - addition to the concentrated waste liquor - is that water in the splitting liquid and residual acid can be economically evaporated in 5-7 steps.
• the addition of residual acid and splitting liquid to the concentrated waste liquor the heating surfaces in the evaporation plant will be less con ⁇ taminated, since saturation conditions are not reached. The need of washing is thus reduced.
• both splitting liquid and residual acid have a considerably lowe dry matter content that the already evaporated black liquor the dry matter content of the liquor to the soda recovery boiler will be lowered, resulting in a reduction in effi- ciency of this boiler.
• the evaporation process is normally performed in 5-7 stages, or even more.
• the heating surfaces in the evaporation stages may consist of tubes or lamellas.
• Fresh vapour is supplied to the first stage and the secondary vapour generated in this is supplied to the second stage, and so on.
• final thickeners are generally used.
• the final thickeners nor ⁇ mally have forced circulation are located parallel to the first and second stages and fresh steam is supplied to the first of the final thickeners.
• the dry matter content range for the final thickeners is normally from ca 55% to
• the entire first stage of the evaporation plant acts as final thickener.
• the dry matter content of liquor supplied to this type of final thickener is generally 40-45% and after this, 65% or more.
• the final thickener according to the falling film principle is divided into two or three sec tions on the liquor side. The sections are mutually inter ⁇ changeable, the section giving the highest dry matter conte before a changeover afterwards receiving liquor with a dry matter content of 40-45%. This procedure enables the heating surfaces to be washed without supplying weaker liquor or water and without loss of evaporation capacity.
• the splitting liquid and/or residual acid is added to the weak liquor before the evaporation process, the content of sodium sulphate in the whole evaporation plant will increase. Even at a dry matter content of 40-45%, the liquor supplied to the final thickener will be very close to or even over the saturation limit for inorganic salts. This means that the washing gives a poor result since the salts can crystallize at this low dry matter content.
• splitting liquid and residual acid are supplied to the final thickener. If there are three sections in the final thickener, these liquids are preferably added to the second and/or third section.
• the first section is in this case the section to which the liquor is supplied.
• the residual acid and splitting liquid can be added separate ly or together, either directly to the circulation system of the final thickener or to the final thickener unit.
• the invention is applicable both to final thickeners with lamellar heating surfaces functioning in accordance with the falling film principle and to final thickeners of the forced circulation type with tubular or lamellar heating surfaces.
• a six-step evaporation plant (215 tons evaporated water per hour) may be stated, with a final three section thickener.
• This final three section evaporator is shown on the drawing.
• I-VI are evaporators (stages) . They are in respect of the conveyance of liquor connected in series in the sequence III, IV, V, I, II and VI.
• VI is the final evapora ⁇ tor (final thickener) .
• A, B and C are the three sections in the final evaporator VI.
• D1 , D2 and D3 are valves for the supply of steam.
• E1 , E2 and E3 are valves for discharging thick waste liquor (fully evaporated liquor) K from
• F1, F2 and F3 are valves for the supply of semi-thickened waste liquor N to, and
• G1 , G2 and G3 are valves for supplying residual solution J when this in accordance with the invention is conducted by the alternative M, to the sections A, B and C respectively.
• H is mixed liquor entering into the evaporator plant, that is waste liquor from the digester house mixed with thick waste liquor K.
• L is the connection alternative which applies for the supp ⁇ ly of residual solution to the evaporator plant in accor ⁇ dance with the conventional process.
• valves are positioned to wash the sections B and C respectively. This is, however, not sufficient but the evaporator once a day has to be shut down to be washed with a weaker liquor at a lower temperature.
• the mixed liquor is added to the evaporator III without being mixed with the residual solu ⁇ tion J.
• the residual solution J is by the connection alternative M conveyed through the valve G2 to the section B of the final e aporator.
• valve timing in the three washing periods required by th three section evaporator VI is controlled by a micro pro ⁇ cessor as follows.
• the temperature drop over the whole six-step plant was lowered by 8-10°C.
• the invention which has been described as applied to the sulphate process, using splitting liquid from splitting of soap skimmings and residual acid from the production of chlorine dioxide to cover the chemical losses, is also applicable when other dilute solutions are used for covering the chemical losses as well as in evaporation of different spent liquors where similar problems are involved, e.g. in evaporating certain types of sulphite waste liquor.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Paper (AREA)
• Vaporization, Distillation, Condensation, Sublimation, And Cold Traps (AREA)
• Physical Or Chemical Processes And Apparatus (AREA)
• Heat Treatment Of Water, Waste Water Or Sewage (AREA)

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Citations (4)

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• 1982
  • 1982-07-12 SE SE8204266A patent/SE8204266L/ not_active Application Discontinuation
• 1983
  • 1983-07-08 PT PT77003A patent/PT77003B/pt unknown
  • 1983-07-11 ES ES524018A patent/ES524018A0/es active Granted
  • 1983-07-11 JP JP58502453A patent/JPS59501218A/ja active Granted
  • 1983-07-11 US US06/598,307 patent/US4533433A/en not_active Expired - Fee Related
  • 1983-07-11 EP EP83902318A patent/EP0113754B1/en not_active Expired
  • 1983-07-11 WO PCT/SE1983/000280 patent/WO1984000390A1/en active IP Right Grant
  • 1983-07-18 CA CA000432590A patent/CA1214003A/en not_active Expired
• 1984
  • 1984-03-08 FI FI840942A patent/FI70942C/fi not_active IP Right Cessation
• 1989
  • 1989-11-07 SE SE8903724A patent/SE467361B/sv not_active Application Discontinuation

Patent Citations (4)

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